The assembly of a semiconductor package requires the packaging of a semiconductor die ("chip") and the interconnection of the electrical contact pads on the die to leads from the package. One such interconnection method is known as C4. "C4" stands for Controlled Collapse Chip Connection. The basic concept of C4 is to connect chips, for example, to leads from chip packages or to traces on multichip module substrates, for example, by means of solder balls partially collapsed between the surfaces of bonding pads on the chips and the leads or traces of the package or multichip module. Tiny balls of electrically conductive solder connect the respective pairs of bonding surfaces (often called "bonding pads" or "pads") on the chip and on the traces. Each pad on the chip has a corresponding pad on the package trace; the pad layouts are mirror images. As the chip is pressed on the traces, the solder balls on the bonding pads of the chip are pressed against the corresponding solderless bonding pads of the traces, partially collapsing the solder balls and establishing electrical connections between the respective bonding pads in one step.
A major application of C4 is in joining semiconductor microchips to chip packages. The C4 balls are placed on chips while they are still joined in a wafer. The chips are made as small as possible to maximize the number of chips that can be obtained per wafer and to increase the chip yield (i.e. the number of good chips obtained divided by the maximum number of chips available from a wafer). For a given defect density, chip yield goes up as chip size goes down. Additionally, the continuous reduction of feature size (submicron feature sizes are now common) by the semiconductor industry requires interconnections at very fine pitches. Therefore, the best C4 fabrication method is the one that allows the placing of thousands of very small, closely-spaced solder balls, each precisely positioned.
C4 solder bumps must be mechanically well-fastened to their bonding pads, or they may be torn off when the two surfaces are pushed together. The attachment of C4 balls requires care.
One method of forming solder balls uses sputtering or vacuum deposition but this method cannot be used for large wafers containing many chips due to limitations on the size of the metal mask used to position the solder bumps. A second method uses electrodeposition to form the solder bumps and also uses a mask to position the solder bumps. However, the electrodeposition method requires a preliminary first step, the creation of a continuous "seed layer" of conductive metal adhered onto the insulating substrate. This metal layer serves to conduct the electrical current which deposits the solder. Alloys such as titanium tungsten (TiW) can be added as barrier layers to prevent intermetallic layers from forming between the solder bump and the bonding pad.
After the seed layer has been laid down, a photoresist mask is formed on the seed layer using a spin on coating technique and the photoresist is then exposed through a mask to light to define the areas where solder bumps are to be formed. Unexposed photoresist where the solder bumps are to be formed is then washed away to leave the cured photoresist behind as a mask.
Following the creation of the photoresist mask with holes where the solder bumps are to be deposited, solder is electrodeposited into the mask holes to form the solder bumps. The photoresist mask is then removed. The seed layer other than under the solder bumps is then removed by etching using a selective etch. Removal of the seed layer electrically isolates the solder bumps. The solder bumps are then reflowed (melted) into solder balls.
In order to provide elongated solder connections between a semiconductor device and a supporting substrate, Lakritz et al. in U.S. Pat. No. 4,545,610 disclose the use of solder extenders. Extensions of solder are formed on a supporting substrate. A semiconductor device having solder mounds corresponding in location to the solder extensions is invertedly placed over the substrate, such that solder extenders and the solder mounds have a direct surface contact. The whole assembly is heated to a temperature sufficient to melt the solder extenders and the solder mounds, thereby forming elongated hourglass shaped solder connections between the device and the substrate.
Allen et al. in U.S. Pat. No. 4,664,309, teach that the life of a solder joint can be increased substantially by a relatively small increase in solder joint height, or by a reduction in solder joint diameter. Allen et al. further disclose that an hourglass-like column-type solder joint provides for more uniform stress and greater flexibility as well as allowing greater packaging density. To this end, Allen et al. describe a mounting device to securely hold preforms of a joint-forming material, such as a solder column, in an aperture.
European Patent Application No. 248,314 discloses the use of a mask, such as a photoresist, to act as a mold for the solder and to thereby produce large solder bump heights.
European Patent Application Publication No. 248,566 discloses heating matching, facing solder bumps on both a package and a substrate so that the contacting solder bumps melt together and coalesce to form an "elongated" solder joint.
Agarwala et al. in U.S. Pat. No. 5,251,806, disclose forming elongated solder interconnections by controlling the collapse of the solder bumps by protecting the solder bumps with encapsulating material. Additional encapsulated solder bumps may be stacked on the first encapsulated layer to provide further elongation of the solder interconnection.
Current conventional processes cannot plate solder bumps at less than a 9 mil pitch for a bump height of 4 mils or more. The processes are limited by the use of the spin-on coating method to apply a photoresist masking material which also acts as a partial mold. The masks produced by spin-on coating are not uniform when the masking material is applied in thicknesses above 2 mils, hence the available mold height using conventional processes is limited to 2 mils. With a mold height limited to 2 mils, solder bumps plated higher than 2 mils are mushroom-shaped and have a poor aspect ratio. The aspect ratio, a convenient parameter used to characterize a solder bump, is defined as the ratio of the volume of the solder bump to the volume of a cylinder having a diameter that is the widest dimension of the solder bump. If the solder bump is a cylinder then this aspect ratio is 1. Typical aspect ratios for prior art solder bumps are significantly less than 1, for example, 0.4 for some mushroom shaped solder bumps. An aspect ratio of 1 for a solder bump indicates that the pitch of a die is limited only by the spacing of the bonding pads since the widest dimension of the solder bump is not appreciably greater than the diameter of the bonding pad.
While the aspect ratio as defined above provides a means for characterizing a solder bump, the aspect ratio is not always a useful characterization. For example, an hourglass shaped solder bump has an aspect ratio of less than 1 but the widest dimension of the solder bump is still not appreciably greater than the diameter of the bonding pad.